Switch language name 'zh' to 'zh-cn'

This is the first step to rename 'zh' to 'zh-cn'. There are several reasons why we rename the language name.

- The upstream docsy theme changed the language name, leading to many warnings during site build;
  The side-effect is that the i18n strings are no longer working.
- We believe renaming the language is the right thing to do, because this move can make room for other variants of Chinese language, such as 'zh-tw', 'zh-sg' etc.

There would be several follow-ups to this PR, such as fixing the intra-site links, adding redirects etc.

We will lock up changes to zh/zh-cn pages for the moment, until this one gets in.

This PR is based on commit cdad0a7342.

content/zh-cn/examples/pods/inject/dependent-envars.yaml

@@ -0,0 +1,26 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: dependent-envars-demo
+spec:
+  containers:
+    - name: dependent-envars-demo
+      args:
+        - while true; do echo -en '\n'; printf UNCHANGED_REFERENCE=$UNCHANGED_REFERENCE'\n'; printf SERVICE_ADDRESS=$SERVICE_ADDRESS'\n';printf ESCAPED_REFERENCE=$ESCAPED_REFERENCE'\n'; sleep 30; done;
+      command:
+        - sh
+        - -c
+      image: busybox:1.28
+      env:
+        - name: SERVICE_PORT
+          value: "80"
+        - name: SERVICE_IP
+          value: "172.17.0.1"
+        - name: UNCHANGED_REFERENCE
+          value: "$(PROTOCOL)://$(SERVICE_IP):$(SERVICE_PORT)"
+        - name: PROTOCOL
+          value: "https"
+        - name: SERVICE_ADDRESS
+          value: "$(PROTOCOL)://$(SERVICE_IP):$(SERVICE_PORT)"
+        - name: ESCAPED_REFERENCE
+          value: "$$(PROTOCOL)://$(SERVICE_IP):$(SERVICE_PORT)"